Nitric acid

Every silver salt in this course begins here. Silver metal is unreactive enough to have been used for coinage, and the reagent that turns it into something a photographer can use is nitric acid. That one reaction is the reason the substance has a page — and the gas it gives off is the reason the page gives no procedure.

In photography

Making silver nitrate. Reilly states the fact in a single clause: “Silver nitrate is formed by the action of nitric acid on silver metal.” Towler’s 1864 manual gives the period method and, more useful to us, the period explanation. Silver in thin laminae or filings, nitric acid and distilled water, in a porcelain or glass dish, warmed gently on a sand bath until the silver disappears; and then the chemistry, in his words: “one portion oxidizes the silver and liberates peroxide of nitrogen; whilst a second combines with the oxide so formed, and produces the nitrate of the oxide of silver.” His peroxide of nitrogen is nitrogen dioxide. Half the acid is not acting as an acid at all; it is acting as an oxidant, and the price of oxidising the silver is a red-brown gas.

⚗The chemistry

This is the distinction that makes nitric acid different in kind from the other acids in this family. Hydrochloric and sulfuric acid attack a base metal by offering protons, and what comes off is hydrogen. Silver is too noble for that, and gives up its electron only to something that will take it — which nitrate, in concentrated acid, will. So the metal is oxidised to Ag⁺, the nitrogen is reduced from its highest oxidation state, and nitrogen dioxide leaves the flask. CAMEO’s reactive-group classification puts nitric acid under Acids, Strong Oxidizing, and records that it ignites on contact with alcohols, amines, ammonia, hydrocarbons, powdered metals and thiols. The oxidising behaviour is not a side effect of the acidity; for photography it is the point.

Ag + 2 HNO₃ → AgNO₃ + NO₂ + H₂O

Towler's account written as an equation: one part of the acid oxidises the silver, one part forms the nitrate, and peroxide of nitrogen is liberated

Keeping a wet-plate silver bath constant. Towler records the practical consequence of that chemistry a century of collodion workers relied on. A cadmium-iodised collodion sensitised in a silver bath “rendered slightly acid with nitric acid, produces irreproachable pictures”, not faster than a bath acidified with acetic acid or made alkaline with carbonate of soda, “but the latter are very unstable, whilst the former remains for a long time constant, and is regarded accordingly the proper bath for the cadmium collodion”. Nitric acid does not decompose the way an organic acid does, so the bath does not drift.

Making gold chloride. Kodak’s 1924 primer records that gold chloride is made by dissolving gold in a mixture of hydrochloric and nitric acids — aqua regia — and evaporating; neither acid will dissolve gold alone. Every gold toner in the formulary descends from that operation.

Measuring a silver bath. Reilly’s adaptation of the Volhard titration, used from at least 1875 to keep track of an albumen paper sensitising bath, acidifies the silver solution with a small quantity of nitric acid so that only the intended reactions take place, and gives the recipe for the 6 mol/L acid it needs. He also records that photographers neutralised ammoniated silver baths with nitric acid, and judges the practice of doubtful value: what it really produced was ammonium nitrate, which accumulates in a bath anyway.

In the older formularies. Wall’s 1924 collection uses a 5 per cent nitric acid bath for five minutes to strip the tenaciously held lead salts out of a gelatin negative after a lead intensifier, and uses the acid to clean glass before silvering it.

Properties

A colourless-to-yellow liquid with a pungent odour that fumes in air, boiling at 121 °C, of relative density 1.4, miscible with water. The colour is diagnostic: NIOSH records that fuming nitric acid is simply concentrated nitric acid containing dissolved nitrogen dioxide, so a yellow or brown bottle is an acid that has begun to decompose. Chemical Safety Card 0183 records that it decomposes on heating, producing toxic nitrogen oxides; that as a strong oxidant it reacts violently with combustible material and reducing agents; that as a strong acid it reacts violently with bases and organic compounds; and that it is corrosive to metals, producing flammable hydrogen. CAMEO records that it reacts violently with water with the production of heat, fumes and spattering. Like the other strong acids it is fully ionised in water, so it has no acid reserve and no buffering: what it offers is a low pH held without drift, which is exactly what a wet-plate bath needed.

Handling

The aggregated ECHA notifications classify it Danger with three pictograms — oxidiser, corrosion and the skull — a combination no other substance in this family carries. H314 appears in more than 99.9 per cent of the 4,215 company reports carrying hazard codes, H272, may intensify fire, in 98.6 per cent, and H331, toxic if inhaled, in 11.9 per cent. NIOSH sets a recommended limit of 2 ppm over eight hours and 4 ppm over fifteen minutes, with an IDLH of 25 ppm; HSE’s EH40 sets no long-term British limit and a short-term one of 1 ppm (2.6 mg/m³). The symptom that decides the classification is in NIOSH’s list and in the safety card: delayed pulmonary oedema. Someone who has breathed nitrogen oxides can feel unremarkable and deteriorate hours later, which removes the ordinary feedback loop by which a darkroom worker knows to open a window.

Why Level C. The course rubric sets Level C for reagents “classified as acutely toxic … at the concentrations used, where a fume cupboard or specialist disposal is the recognised control”, and for any procedure whose waste cannot lawfully enter a domestic drain. Both apply. The acid is classified toxic by inhalation; the reaction photography actually wants from it evolves a gas whose harm is delayed; it is simultaneously an oxidiser that ignites organic materials on contact; and its waste is acidic and metal-bearing. The controls the rubric names for Level C are the ones this substance needs — engineered extraction, a written procedure, a second competent person, and a licensed waste route.

⚠Safety

This course does not give a procedure for dissolving silver in nitric acid, and buys silver nitrate instead. The reasons are the three above, and one more: the operation is most tempting at exactly the moment it is least safe, when a printer has accumulated silver-bearing waste and would like to recover it. Recovery by electrolysis or by metallic replacement, which Kodak’s own publications describe, does not need this acid. If you ever meet nitric acid in a supervised laboratory, the handling rules are Chemical Safety Card 0183’s: acid into water and never water into acid, printed on the card in capitals; a face shield with breathing protection; nothing combustible, no alcohol, no ammonia, no acetic acid and no metal anywhere near it; and the whole operation inside extraction, because the hazard you cannot smell in time is the one that matters.

Waste

The honest position is that this course produces no nitric acid waste, because it gives no procedure that makes any. Where a supervised laboratory does, the stream is acidic, oxidising and usually carries dissolved metal, and it falls squarely under the rubric’s Level C criterion of waste that cannot lawfully enter a domestic drain and needs a licensed contractor. Kodak’s J-52 publication gives 5.6 to 9.4 as the pH window sewer codes most frequently set, with 1.2 mg/L for silver and 17 mg/L for iron among the metals most often limited; a solution of silver in nitric acid fails on pH and on metal together. Silver is recovered first, because it is both valuable and regulated, and the remaining acid goes out by the licensed route.

Two mixing rules apply absolutely. Nothing organic — no paper towel, no sawdust, no alcohol — is used to absorb or dilute it. And it is never poured into a container of other waste, because an oxidising acid meeting a sulfide, a cyanide or a thiosulfate produces a gas rather than a solution. Check your local regulations, which govern, and where a jurisdiction requires a licensed contractor, that is the whole answer.

History

Nitric acid is in photography’s founding story twice over, and both times as the acid that carried silver. Johann Heinrich Schulze’s chalk, in 1727, was moistened with aqua fortis — nitric acid kept for parting silver from gold, and carrying dissolved silver as an impurity — and it was that accident that produced the first image made by light. A century later Frederick Scott Archer’s collodion began with the same acid used quite differently: nitric acid acting on cotton fibre to make pyroxylin, which dissolved in ether and alcohol gave the film that held the halide on the wet plate. Archer’s own instruction on that operation reads like a Level C classification written before the phrase existed. It must be conducted “either in the open air, or in some convenient situation where there is sufficient draught to carry off the nitric acid vapour generated”, and the washing afterwards must be so thorough that “not a trace of acid should be left”.

The nineteenth century made its own silver nitrate because there was often no one to buy it from. Towler’s chapter is addressed to a photographer refining coinage silver at the bench. What has changed is not the chemistry but the supply: silver nitrate is a catalogue item, and the one operation that made this acid indispensable to a photographer no longer has to be done by one.